I recently overhauled bits of the mail system here to take care of a few lingering quirks that I’d never had the time nor inclination to track down. All of my various email addresses and aliases go to the exact same mailbox, through the multiple expedients of fetchmail, which picks up my mail from gmail and AOL, and DNS MX records that point everything to the same place.
Until recently, if you sent mail to “firstname.lastname@example.org” it would be transformed by the server into “email@example.com” unceremoniously. It would show up that way in the mailbox, and only by delving into the mail headers was it obvious that the mail was originally destined for a different domain. For addresses I didn’t make use of much, this was fine, though it leads to the curious circumstance where somebody sending mail to goodjobsucking.com would get replies from baddomain.com, which deviates from the principles of separating domains in the first place.
It turns out the root cause was that goodjobsucking.com, rather than having its own A record in the DNS tables, used a CNAME to baddomain.com. Apparently this implies that mail sent to goodjobsucking.com is actually for baddomain.com. I imagine this would be particularly useful for adjunct or typo domains, where you want to correct the original destination or transition from one domain to another. It’s also useful in that the mailer only needs to internally relay for, and listen to, mail destined for baddomain.com; any mail sent to a CNAME from another domain pointing to it works perfectly well.
Moving the domain from a CNAME to an A record effectively separates things out again, though now the mailer must also be aware that it’s listening for mail for yet another domain.